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Moon and Spica

January 5, 2021

You don’t need big telescopes or special instruments to learn about the universe. Sometimes, you can learn a great deal with not much more than your eyes alone.

2200 years ago, for example, the Greek astronomer Hipparchus learned that the stars change position relative to the Sun from year to year. He did so by measuring the position of Spica, the brightest star of Virgo. Spica is in good view at dawn tomorrow, to the lower left of the last-quarter Moon.

During a lunar eclipse, Hipparchus measured the angle from Spica to the middle of the Moon. And from that, he calculated Spica’s position relative to the Sun. He found that the star had moved about two degrees since another eclipse 150 years earlier — about the width of a finger held at arm’s length. He realized that the entire celestial sphere — the background of fixed stars — rotated with respect to the Sun.

That rotation is known as the precession of the equinoxes. It’s caused not by the stars, but by Earth. Our planet “wobbles” on its axis like a spinning gyroscope that’s running down. As it wobbles, the stars appear to shift position relative to the Sun. It takes about 26,000 years to complete one full wobble and have the stars return to their starting positions.

The precession of the equinoxes was an important discovery because it showed that the heavens can change — a discovery made with some simple tools and the nimble mind of Hipparchus.

Script by Damond Benningfield

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